33
\$\begingroup\$

In chat, we are often fast-typers and don't really look at the order of letters before posting a message. Since we are lazy, we need a program that automatically swaps the last two letters in our words, but since we don't want to respond too late, the code must be short.

Your task, if you wish to accept it, is to write a program that flips the last two letters of each word in a given string (so the word Thansk turns into Thanks). A word is a sequence of two or more letters in the English alphabet delimited by a single space.

  • The string / list of characters you receive as input is guaranteed to only contain alphabetic characters and spaces (ASCII [97 - 122], [65 - 90] and 32).

  • You can take input and provide output through any standard method, in any programming language, while taking note that these loopholes are forbidden by default.

  • The output may have one trailing space and / or one trailing newline.

  • The input will always contain words only (and the corresponding whitespace) and will consist of at least one word.

This is code-golf, so the shortest submission (scored in bytes), in each language wins!

Test cases

Note that the strings are surrounded with quotes for readability.

Input -> Output

"Thansk"                                 -> "Thanks"
"Youer welcoem"                          -> "Youre welcome"
"This is an apple"                       -> "Thsi si na appel"
"Flippign Lettesr Aroudn"                -> "Flipping Letters Around"
"tHe oDd chALlEneg wiht swappde lettesR" -> "teH odD chALlEnge with swapped letteRs"

Or, for test suite convenience, here are the inputs and their corresponding outputs separately:

Thansk
Youer welcoem
This is an apple
Flippign Lettesr Aroudn
tHe oDd chALlEneg wiht swappde lettesR
Thanks
Youre welcome
Thsi si na appel
Flipping Letters Around
teH odD chALlEnge with swapped letteRs

Thanks to DJMcMayhem for the title. This was originally a CMC.

\$\endgroup\$
  • \$\begingroup\$ May we output an array of words? \$\endgroup\$ – Shaggy Dec 27 '17 at 22:03
  • \$\begingroup\$ @Shaggy No, the output must be a string (or a list of characters by default) \$\endgroup\$ – Mr. Xcoder Dec 27 '17 at 22:04
  • \$\begingroup\$ May we request a trailing space on each input? \$\endgroup\$ – FlipTack Dec 29 '17 at 12:23
  • \$\begingroup\$ @FlipTack It was allowed in the initial version, but I have removed that rule before any of the answers that would use that had been posted. (partly because some users in chat told me I'm making this too easy otherwise, and I agree with them). No, it's not allowed. \$\endgroup\$ – Mr. Xcoder Dec 29 '17 at 12:25
  • 1
    \$\begingroup\$ @Fabian A word is a sequence of two or more letters \$\endgroup\$ – Mr. Xcoder Dec 29 '17 at 14:12

39 Answers 39

16
\$\begingroup\$

V, 4 5 bytes

òeXp

Try it online!

|| denotes the cursor

The buffer starts with |w|ord and more words and the cursor being on the first character.

Recursively ò

go to the end of a word

wor|d| and more words

remove X the character to the left of the cursor

wo|d| and more words

paste it over the next character

wod|r| and more words

Implicit ending ò, repeat the same process for other words until the end of the buffer is reached

\$\endgroup\$
10
\$\begingroup\$

Jelly, 7 bytes

Ḳœ?@€2K

A monadic link taking and returning lists of characters

Try it online!

How?

Ḳœ?@€2K - Link: list of characters
Ḳ       - split at spaces
     2  - literal two
    €   - for €ach:
   @    -   with sw@pped arguments:
 œ?     -     nth permutation (the 2nd permutation has the rightmost elements swapped)
      K - join with spaces
\$\endgroup\$
  • \$\begingroup\$ That's a nice abuse of permutations. Alternative \$\endgroup\$ – Mr. Xcoder Dec 27 '17 at 20:21
  • \$\begingroup\$ @Mr.Xcoder Ḳ2œ?ЀK also works and uses a single quick. \$\endgroup\$ – Dennis Dec 28 '17 at 3:40
7
\$\begingroup\$

Brain-Flak, 122 bytes

{(({})[((((()()){}){}){}){}])((){[()](<{}>)}{}){{}<>(({}({}))[({}[{}])])(<>)}{}({}<>)<>}<>(({}({}))[({}[{}])]){({}<>)<>}<>

Try it online!

The worst language for the job :)

Readable Slightly more readable version:

{
    (({})[((((()()){}){}){}){}])((){[()](<{}>)}{})

    {
        {}
        <>

        (({}({}))[({}[{}])])

        (<>)
    }
    {}

    ({}<>)<>

}<>

(({}({}))[({}[{}])])

{

    ({}<>)
    <>
}<>
\$\endgroup\$
  • \$\begingroup\$ I can't believe this is longer than the Brainfuck version... \$\endgroup\$ – Pureferret Dec 28 '17 at 12:46
  • \$\begingroup\$ @pureferret Brain-flak tends to be longer than brainfuck. Mostly cause it requires two bytes per primitive command, where brain-flak requires two. \$\endgroup\$ – DJMcMayhem Dec 28 '17 at 13:21
7
\$\begingroup\$

Haskell, 40 bytes

(f=<<).words
f[a,b]=b:a:" "
f(x:r)=x:f r

Try it online! Usage example: (f=<<).words $ "abc xyz" yields "acb xzy ".

\$\endgroup\$
  • \$\begingroup\$ So you're telling me the shortest approach is both the approaches combined? >_< \$\endgroup\$ – totallyhuman Dec 27 '17 at 21:16
7
\$\begingroup\$

Retina, 13 bytes

(.)(.)\b
$2$1

Try it online! Link includes test cases.

\$\endgroup\$
6
\$\begingroup\$

Python 3, 50 bytes

print(*(w[:-2]+w[:-3:-1]for w in input().split()))

Try it online!

This answer abuses Python 3's behavior of print: Multiple arguments are printed with a single space between them. Of course, we can't just give it multiple arguments because we don't know how many words will be in the input. So we use the splat operator. Basically

print(*[a,b,c])

is exactly the same thing as

print(a,b,c)

Abusing that makes a full program turn out shorter than a function/lambda where we'd have to use ' '.join or something similar.

\$\endgroup\$
  • \$\begingroup\$ Looks like Python 2 saves 2 bytes by writing for w in input().split():print w[:-2]+w[:-3:-1],. In Python 3, extracting the last two characters would work well with print(*(''.join(a)+c+b for*a,b,c in input().split())) except that a needs to be remade into a string. \$\endgroup\$ – xnor Dec 28 '17 at 4:01
5
\$\begingroup\$

Matlab (R2016b), 51 50 bytes

Saved 49 50 (!) bytes thanks to @Giuseppe.

function s(a),regexprep(a,'(\w)(\w)( |$)','$2$1 ')

And my previous answer:

Matlab (R2016b), 100 bytes

(Just for the fun of it :P)

function s(a),a=regexp(a,' ','split');for i=1:length(a),fprintf('%s ',a{i}([1:end-2 end end-1])),end

Explanation:

function s(a) % Defining as a function...
a=regexp(a,' ','split'); % Splits the input string at the spaces
for i=1:length(a) % Loops through each word
    fprintf('%s ',a{i}([1:end-2 end end-1])) % And prints everything with the last two characters swapped.
end
\$\endgroup\$
  • 1
    \$\begingroup\$ one character words can't happen, as a word is defined to be at least two characters. \$\endgroup\$ – Giuseppe Dec 27 '17 at 20:30
  • \$\begingroup\$ would regexprep work here? Something like regexprep(a,'(\w*)(\w)(\w)','\1\3\2')? \$\endgroup\$ – Giuseppe Dec 27 '17 at 20:53
  • \$\begingroup\$ D= This. Was. Epic! I think you should post this answer, since it's totally different from mine. The only thing is that Matlab references the matches with $1, and not \1, so it would be regexprep(a,'(\w*)(\w)(\w)','$1$3$2'). \$\endgroup\$ – Thiago Oleinik Dec 27 '17 at 20:59
  • 1
    \$\begingroup\$ you should post it as a separate answer / in this answer; it's always good to see if a regex would help or not on a string challenge! Besides, I clearly don't understand MATLAB's regex engine, so it wouldn't quite be fair for me to take credit for it. \$\endgroup\$ – Giuseppe Dec 27 '17 at 21:02
  • 1
    \$\begingroup\$ function s(a),regexprep(a,'(\w)(\w)( |$)','$2$1 ') is still another byte shorter! \$\endgroup\$ – Giuseppe Dec 27 '17 at 23:30
5
\$\begingroup\$

C,  62   58  54 bytes

Thanks to @Dennis for saving  four  eight bytes!

f(char*s){s[1]>32||(*s^=s[-1]^=*s^=s[-1]);*++s&&f(s);}

Try it online!

\$\endgroup\$
  • \$\begingroup\$ ah, the xor-based swap \$\endgroup\$ – Snowbody Dec 29 '17 at 14:52
4
\$\begingroup\$

Prolog (SWI), 60 bytes

[A,B]+[B,A].
[A,B,32|U]+[B,A,32|Y]:-U+Y,!.
[A|U]+[A|Y]:-U+Y.

Try it online!

Explanation

First we define the base case:

p([A,B],[B,A]).

This means that the last two letters will always be swapped.

Then we define what happens if we are right next to a space:

p([A,B,32|U],[B,A,32|Y]):-p(U,Y),!.

Two strings match if right before a space the letters before the space are swapped and the remainder if the strings match. We then use ! to cut.

Our last case is if we are not next to a space the first two letters need to match.

p([A|U],[A|Y]):-p(U,Y).
\$\endgroup\$
4
\$\begingroup\$

Wolfram Language, 117 bytes

StringReplace[RegularExpression["\\b[[:alpha:]]{2,}\\b"]:>StringDrop[StringInsert["$0",StringTake["$0",{-1}],-3],-1]]

Try it online!

Applied to the test strings.

StringReplace[
  RegularExpression["\\b[[:alpha:]]{2,}\\b"] :> 
   StringDrop[StringInsert["$0", StringTake["$0", {-1}], -3], -1]] /@
 {"Thansk", "Youer welcoem", "This is an apple", 
  "Flippign Lettesr Aroudn", "tHe oDd chALlEneg wiht swappde lettesR"} // Column
Thanks
Youre welcome
Thsi si na appel
Flipping Letters Around
teH odD chALlEnge with swapped letteRs
\$\endgroup\$
  • 4
    \$\begingroup\$ Welcome to PPCG! \$\endgroup\$ – Steadybox Dec 27 '17 at 21:06
  • \$\begingroup\$ @Steadybox Thanks. \$\endgroup\$ – Edmund Dec 27 '17 at 23:25
4
\$\begingroup\$

R, 111 51 41 bytes

Courtesy of @Giuseppe, a regex approach which blows my old method out of the water.

cat(gsub("(.)(.)\\b",'\\2\\1',scan(,"")))
\$\endgroup\$
  • 1
    \$\begingroup\$ regex are much more efficient here: Try it online! \$\endgroup\$ – Giuseppe Dec 27 '17 at 20:48
  • \$\begingroup\$ (not that I don't appreciate the guts it takes to do a pure string manipulation approach in R) \$\endgroup\$ – Giuseppe Dec 27 '17 at 20:50
  • \$\begingroup\$ 84 byte golf of your approach \$\endgroup\$ – Giuseppe Dec 27 '17 at 23:22
  • \$\begingroup\$ @Giuseppe Wow, nice work! I've edited them into my answer, although if you'd prefer to make your own answer please go ahead! \$\endgroup\$ – rturnbull Dec 28 '17 at 1:10
  • 1
    \$\begingroup\$ nah, don't worry about it. I golfed down another 10 bytes: porting another regex approach, and a 70 byte of your old approach \$\endgroup\$ – Giuseppe Dec 28 '17 at 15:14
4
\$\begingroup\$

APL (Dyalog Classic), 28 bytes

1↓∊((¯2↓⊢),2↑⌽)¨' '(,⊂⍨⊣=,)⍞

⎕ML and ⎕IO are both 1,

Try it online!

Explanation

  • ... (,⊂⍨⊣=,) ... Split (while keeping borders, and appending a border to the beginning) ...
  • ... ⍞ ... the input ...
  • ... ' ' ... ... at spaces.
  • ... ( ... )¨ ... Then, to each element of that:
    • ... , ... Concatenate ...
    • ... (¯2↓⊢) ... ... every item except the last two ...
    • ... 2↑⌽ ... ... with the reverse of the last two elements.
  • 1↓∊ ... Finally, return all but the first element of the flattened result.
\$\endgroup\$
  • \$\begingroup\$ return all but the first \$\endgroup\$ – Adám Feb 1 '18 at 11:18
3
\$\begingroup\$

Funky, 34 bytes

s=>s::gsub("(.)(.)( |$)","$2$1$3")

Try it online!

\$\endgroup\$
3
\$\begingroup\$

Haskell, 45 bytes

-2 bytes thanks to H.PWiz.

(r.g.r=<<).words
g(x:y:z)=' ':y:x:z
r=reverse

Try it online!

\$\endgroup\$
3
\$\begingroup\$

J, 20 19 11 bytes

Credit to @Bolce Bussiere

1&A.&.>&.;:

Try it online!

       &.;:      on words
    &.>          on each
  A.             apply the permutation
1&               number 1, swap the last two elements
\$\endgroup\$
  • 1
    \$\begingroup\$ 13 Bytes with (1&A.&.>)&.;: \$\endgroup\$ – Bolce Bussiere Dec 31 '17 at 19:24
  • \$\begingroup\$ @BolceBussiere perfect \$\endgroup\$ – FrownyFrog Jan 1 '18 at 18:27
  • \$\begingroup\$ Could you add an explanation? Wondering if I can port it to K to reduce the embarrassing byte count of my solution! \$\endgroup\$ – streetster Jan 1 '18 at 21:18
3
\$\begingroup\$

Alice, 24 bytes

/0RR'.%$1\' o
\ix*o ne@/

Try it online!

Explanation

/...\' o
\.../

This forms a loop where the loop body is a linear Ordinal snippet and we execute ' o in Cardinal mode between every two loop iterations. The latter just prints a space.

Unfolding the zigzag structure of the Ordinal code, the linear loop body actually looks like this:

iR*' %e10xRo.n$@

Breaking this down:

i     Read all input. On subsequent iterations, this will push an empty string.
R     Reverse.
*     Join. On the first iteration, this joins the input to an implicit empty string,
      which does nothing. On subsequent iterations, it will join the empty string to
      the word on top of the string, thereby getting rid of the empty string.
' %   Split around spaces. On the first iteration, this will split the input
      into individual words. On subsequent iterations, this does nothing.
e10   Push "10".
x     Use this to permute the (reversed) word on top of the stack. In
      particular, the word is rearranged with the same permutation that is
      required to sort the string "10", which means the first two letters
      get swapped (which correspond to the last two letters of the actual
      word).
R     Reverse the swapped word.
o     Print it.
.n$@  If there are no words left on the stack, terminate the program.
\$\endgroup\$
  • \$\begingroup\$ Just noticed that the letter swap can be done in three bytes (h~Z) instead of four (e10x), but I'm not seeing a way to adjust the layout to actually save a byte overall with that. \$\endgroup\$ – Martin Ender Jan 3 '18 at 13:52
2
\$\begingroup\$

brainfuck, 109 100 bytes

Edit: don’t have to handle one letter words

,[>++++[-<-------->],]>+[-<[>++++[<++++++++>-]<[->>+<<]<]<<[->>+<<]>[[-<+>]>]<<[>+>+>]-<]>>>>>>>[.>]

Try it online!

Prints a trailing space

How It Works

,[>++++[-<-------->],] Puts input on the tape and subtracts 32 from each character
                       This separates each word

>+[- Start the loop
   <[>++++[<++++++++>-]<[->>+<<]<] Add 32 to each letter of the word
                                   Skip this on the first iteration for the last word

   <<[->>+<<]>[[-<+>]>] Swaps the last two letters of the word
   <<[>+>+>]- If there is another word to the left continue loop
              Also set up to add a space to the end of the word
 <] End loop
 >>>>>>>[.>] Print the modified string

Previous version, 109 bytes

,[>++++[-<-------->],]>+[-<[>++++[<++++++++>-]<[->>+<<]<]<<[[->>+<<]>[[-<+>]>]<<[<]]>[>]<[>+>+>]-<]>>>>>>[.>]

Try it online!

\$\endgroup\$
2
\$\begingroup\$

QuadR, 8 bytes

..\b
⌽⍵M

Try it online!

\$\endgroup\$
1
\$\begingroup\$

PHP, 119 107 bytes

Edit: thanks to totallyhuman

<?php foreach(explode(" ",trim(fgets(STDIN)))as$w)echo substr($w,0,strlen($w)-2).strrev(substr($w,-2))," ";

Try it online!

\$\endgroup\$
  • 1
    \$\begingroup\$ Can't you make $word a single character variable name? \$\endgroup\$ – totallyhuman Dec 28 '17 at 0:33
  • \$\begingroup\$ @totallyhuman Yup! I wrote the full version and then compressed it, but didn't notice that. Thanks you. \$\endgroup\$ – Zerquix18 Dec 28 '17 at 3:33
  • \$\begingroup\$ PHP open tags can be omitted in the answer saving you 6 bytes. \$\endgroup\$ – Daniel W. Dec 28 '17 at 12:26
  • \$\begingroup\$ I wonder if fgets(STDIN) can be omitted or replaced by $x too, like not all answers do count the input to their answers \$\endgroup\$ – Daniel W. Dec 28 '17 at 12:32
  • \$\begingroup\$ trim() should be unnecessary. \$\endgroup\$ – Titus Dec 29 '17 at 8:55
1
\$\begingroup\$

Haskell, 41 bytes

foldr(%)" "
a%(b:' ':r)=b:a:' ':r
a%s=a:s

Try it online!

Outputs with a trailing space.

The repeated ' ':r looks wasteful. But a%(b:t@(' ':r))=b:a:t is the same length and a%(b:t)|' ':_<-t=b:a:t is one byte longer.


Haskell, 41 bytes

f(a:b:t)|t<"A"=b:a:f t|1>0=a:f(b:t)
f e=e

Try it online!

\$\endgroup\$
1
\$\begingroup\$

sed, 20 17+1 (-r) = 18 bytes

s/(.)(.)\b/\2\1/g

Try it online!

\$\endgroup\$
  • \$\begingroup\$ The TIO link does not match your posted code. The TIO link is a few bytes longer. \$\endgroup\$ – Xcali Dec 28 '17 at 3:06
  • \$\begingroup\$ Whoops, fixed the link \$\endgroup\$ – Noskcaj Dec 28 '17 at 3:14
  • \$\begingroup\$ You can remove |$. It's not doing anything. (For it to do what you expect you'd need (.)(.)(\b|$), but that's not necessary because \b already matches the end of the string.) \$\endgroup\$ – Jordan Dec 28 '17 at 19:42
  • \$\begingroup\$ Whoops, meant to get rid of that. Thanks, \$\endgroup\$ – Noskcaj Dec 28 '17 at 19:58
1
\$\begingroup\$

PHP, 65 bytes

requires PHP 7.1 (or later)

for(;$s=$argv[++$i];$s[-1]=$s[-2],$s[-2]=$c,print"$s ")$c=$s[-1];

takes sentence as separate command line arguments. Run with -nr.


working on a single string, 77+1 bytes:

foreach(explode(" ",$argn)as$s){$c=$s[-1];$s[-1]=$s[-2];$s[-2]=$c;echo"$s ";}

Run as pipe with -nR.


... or try them online.

\$\endgroup\$
1
\$\begingroup\$

Java 8, 35 bytes

s->s.replaceAll("(.)(.)\\b","$2$1")

Port of @TaylorScott's Google Sheets answer, after I golfed two bytes. EDIT: I see it's now a port of Neil's Retina answer after my two golfed bytes.

Explanation:

Try it online.

s->                           // Method with String as both parameter and return-type
   s.replaceAll("(.)(.)       //  Replace any two characters,
                       \\b",  //  with a word-boundary after it (space or end of String)
                "$2$1")       //  With the two characters swapped
\$\endgroup\$
1
\$\begingroup\$

Google Sheets, 33 Bytes

Anonymous worksheet function that takes input from cell A1 and outputs to the calling cell

=RegExReplace(A1,"(.)(.)\b","$2$1

-2 Bytes Thanks to @KevinCruijssen for the use of (.) over (\w)

\$\endgroup\$
  • \$\begingroup\$ Both (\w) can be golfed to (.) if I'm not mistaken. The \b is already an indication to look for words only. (Not entirely sure though, but it works in Java.) \$\endgroup\$ – Kevin Cruijssen Feb 1 '18 at 10:54
  • \$\begingroup\$ @KevinCruijssen - You are absolutely correct, it can be. Thank you! \$\endgroup\$ – Taylor Scott Feb 1 '18 at 17:52
1
\$\begingroup\$

JavaScript (Node.js), 38 36 32 bytes

s=>s.replace(/(.)(.)( |$)/g,"$2$1 ") 
s=>s.replace(/(.)(.)\b/g,"$2$1")

Try it online!

RegExp approach courtesy @Giuseppe (although I thought of this independently), assuming words separated by only one space

-2 for only considering 1 space and add trailing space

-4 Thanks @Shaggy

\$\endgroup\$
  • \$\begingroup\$ Doesn't matter if there are more spaces, I think \$\endgroup\$ – l4m2 Dec 28 '17 at 9:48
  • \$\begingroup\$ @l4m2 But if there are more spaces then it will become a 38 for s=>s.replace(/(.)(.)( +|$)/g,"$2$1$3"). \$\endgroup\$ – Shieru Asakoto Dec 28 '17 at 9:49
  • \$\begingroup\$ @l4m2 BTW my original answer was s=>s.replace(/(.)(.)(\s|$)/g,"$2$1$3") \$\endgroup\$ – Shieru Asakoto Dec 28 '17 at 9:51
  • \$\begingroup\$ ab abc abcd abcde abcdef does ab_, bc_, cd_, de_, ___, ef_, ___ \$\endgroup\$ – l4m2 Dec 28 '17 at 9:55
  • 1
    \$\begingroup\$ F=s=>s.replace(/(.)(.)(?!\w)/g,"$2$1") same length \$\endgroup\$ – l4m2 Dec 28 '17 at 10:08
1
\$\begingroup\$

K (oK), 23 22 bytes

" "/{x@prm[!#x]1}'" "\

Try it online!

Example:

" "/{x@prm[!#x]1}'" "\"Hello World"
"Helol Wordl"

Explanation:

Port of FrownyFrog's solution to save 1 byte.

I'll come back to this.

" "/{prm[x]1}'" "\ / the solution
              " "\ / split input on " "
    {       }'     / apply lambda to each
     prm[x]        / permute input x
           1       / and take the 2nd result
" "/               / join with " "

Previous solution:

  • " "/-2{(x_y),|x#y}'" "\ 23 bytes
\$\endgroup\$
1
\$\begingroup\$

05AB1E, 7 bytes

#vy`sðJ

Try it online!

-1 thanks to Magic Octopus Urn.

Prints one trailing space.

\$\endgroup\$
  • \$\begingroup\$ This is 11 bytes \$\endgroup\$ – Daniel W. Dec 28 '17 at 12:50
  • 2
    \$\begingroup\$ @DanFromGermany No, 05AB1E has a code page in which this can be represented as 8 bytes. \$\endgroup\$ – Erik the Outgolfer Dec 28 '17 at 12:51
  • \$\begingroup\$ Can you run the program represented in 8 bytes? \$\endgroup\$ – Daniel W. Dec 28 '17 at 13:06
  • \$\begingroup\$ @DanFromGermany Yes, the 05AB1E interpreter can run this program from a file in the 05AB1E encoding. \$\endgroup\$ – Erik the Outgolfer Dec 28 '17 at 13:09
  • 1
    \$\begingroup\$ @MagicOctopusUrn It's not a list though, it's after `. \$\endgroup\$ – Erik the Outgolfer Feb 2 '18 at 14:58
0
\$\begingroup\$

Jelly, 10 bytes

ḲṪ,¥\€Ḋ€UK

Try it online!

\$\endgroup\$
0
\$\begingroup\$

SNOBOL4 (CSNOBOL4), 136 119 bytes

	I =INPUT
B	I SPAN(&LCASE &UCASE) . Y ARBNO(' ') =:F(O)
	Y RPOS(2) REM . Z =REVERSE(Z)
	O =O Y ' '	:(B)
O	OUTPUT =O
END

Try it online!

Prints with a trailing space. You know you've done something wrong when a language is a backronym for StriNg Oriented and symBOlic Language and your code is longer than Brain-Flak :( now it's slightly better.

Line B takes I and replaces (alphabetic characters saved as Y)(some number of spaces) with the empty string.

The following line extracts the last 2 characters of Y as Z and replaces them as Z reversed, then the next line concatenates O, Y, and a single space character.

Finally, it prints when I no longer matches the required pattern in line B.

\$\endgroup\$
0
\$\begingroup\$

Perl 5, 19 + 1 (-p) = 20 bytes

s/(\w)(\w)\b/$2$1/g

Try it online!

\$\endgroup\$

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